Recently, there has been an increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them.
In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Many secondary batteries are currently available. Among these, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries.
Generally, a lithium secondary battery is prepared by using a cathode and an anode which are each made of a material capable of intercalating and disintercalating lithium ions, and filling an organic or polymer electrolyte solution between the cathode and the anode, and the battery produces electrical energy by oxidation and reduction when the lithium ions are intercalated and disintercalated in the cathode and the anode.
In lithium secondary batteries which are currently available, an anode is mostly made of carbon-based materials as an electrode active material. Particularly, graphite which has been commercially available has a real capacity of about 350 to 360 mAh/g which approaches its theoretical capacity of about 372 mAh/g. Although a carbon-based material such as graphite has a capacity of such a degree, it does not meet the demand for high-capacity lithium secondary batteries as an anode active material.
In order to meet such a demand, attempts have been made to use metals as an anode active material, for example, Si, Sn, an oxide and an alloy thereof, that have a higher charge/discharge capacity than the carbon materials and allow electrochemical alloying with lithium. However, this metal-based electrode active material has a great change in volume during charging/discharging, which may cause cracks and micronization to the active material. Secondary batteries using this metal-based anode active material may suddenly be deteriorated in capacity and have reduced cycle life during repeated charging/discharging cycles. Accordingly, there is a need for solving the deterioration of capacity and cycle life by the use of such a metal-based electrode active material.
In particular, SiO is conventionally used by mixing it with graphite and dispersing a conductive material to the mixture as it has low initial efficiency, electric conductivity and life characteristic. However, the conductive material is not uniformly distributed on the surface of SiO due to its poor dispersity, and such an ununiform distribution may greatly deteriorate the cycle performance of batteries. In order to solve this problem, there is an attempt of pre-treating carbon nanotube (CNT) on the surface of SiO to improve the life characteristic of batteries, but the CNT has not been well-attached to the surface of SiO.